A typical solenoid based anti-lock braking system (ABS), utilizes two solenoid valves per ABS control channel. One of the two solenoid valves is of the common normally open type, and the other is of the common normally closed type. When employed on a passenger vehicle, ABS is generally configured with three or four control channels so that a total of six or eight solenoid valves are required. In addition, two hydraulic pumps are typically used to move fluid through the three or four control channels. Two pump inlet accumulators, and two pump outlet damper assemblies are also included in the common fully operational system. This arrangement provides for maximum performance by enabling the wheel pressures to be individually regulated in various ways, and provides a system that operates in increasing pressure, holding pressure, and decreasing pressure states. Holding pressure states are made possible by employing both the normally open and normally closed valves and are typically required to temporarily maintain wheel brake pressure during an ABS cycle. The pump inlet accumulator is able to temporarily store fluid during a given release cycle, and is therefore, necessary for achieving acceptably high release rates for decreasing pressure during an ABS release cycle. However, this arrangement tends to be relatively costly since two independent valves and corresponding electrical drive circuits are required for operation of each ABS control channel. Sufficient space must also be allocated for packaging of the pump inlet accumulators. Additionally, the use of two accumulators and the structure of these prior art systems further increases cost.
As the use of ABS has proliferated in vehicle braking systems, further emphasis has been placed on driving the cost and space requirements out of the typical dual-solenoid per control channel system. Cost savings have been achieved to some extent by continuously downsizing and integrating system components such as the ABS control valve block, ABS pumps and ABS accumulators into a small size which can be packaged relatively easily underhood.
The art has also identified proportional pressure control solenoid valves as potentially useful in moving from dual solenoid per control channel ABS, to a multi-port, single flow control valve per control channel system. With this type of proportional solenoid valve, during ABS operation the single flow control valve feeds a controlled pressure to the wheel brake(s) by diverting excess pressure internally within the valve from the wheel brake to an accumulator. With the theory of proportional solenoid control now being well-known, mass produced systems of the type are conspicuously absent in passenger vehicle applications. This can be generally attributed to the undesirable complexity and cost of such a system. Additional approaches that purport to operate with a single valve per ABS control channel incorporate wheel dependencies that can be undesirable and which increase algorithm and system complexity adding to cost. Accordingly, a low cost alternative to the presently common dual solenoid ABS system remains highly desirable in the very competitive ABS braking system marketplace.